1. Field of the Invention
The present invention relates to a magnetic recording medium such as magnetic recording drums, magnetic recording tapes, magnetic recording disks, magnetic recording cards, and so on, and a magnetic storage apparatus. More particularly, the invention relates to a perpendicular magnetic recording medium suitable for ultra-high density magnetic recording of 50 gigabits or more per 1 square inch, and a magnetic storage apparatus using the same.
2. Description of the Related Art
With the popularized use of the Internet in recent years, there have been increases not only in the shipping number of personal computers, but also in the demand for a magnetic recording disk device. Connection to the Internet can be made not only by a personal computer but also by a portable terminal. However, to make the portable terminal itself more convenient, it is essential to incorporate the magnetic recording disk device, and the demand in this field is also expected to grow in the future. In addition, as digital television broadcasting is near at hand, the use of the magnetic recording disk device as a video recorder has begun on a full scale. Accordingly, the application field of the magnetic recording disk device has been increasingly expanded. Still, however, further miniaturization and a larger capacity have been requested by users.
The magnetic recording disk device thus far available has employed an in-plane recording system. In in-plane recording, the recording direction of magnetization is in-plane, and adjacent magnetization is reverse in polarity. Thus, between adjacent recording bits, a magnetization transition region is formed in order to reduce magnetostatic energy. A large width of the magnetization transition region causes an increase in noise and, thus, to reduce noise, thin formation of a magnetic layer and micronization of the magnetic crystal grain size are considered to be effective. Therefore, an approach to the high recording density of an in-plane recording medium is to focus on how to reduce the volume of a very small magnet constituting a recording bit. It is generally considered, however, that it will be difficult to deal with a much higher recording density expected in the future, with the in-plane recording medium, because of a physical limitation. In other words, the in-plane recording medium may have a problem in basic performance for saving recording information, due to the thermal fluctuation phenomenon of magnetization following the micronization of the very small magnet constituting the recording bit.
For the foregoing reason, a perpendicular magnetic recording medium has again attracted attention recently. In perpendicular recording, the direction of recorded magnetization is perpendicular to a film plane, and no strong charge is present between adjacent recording bits, preventing the width of a magnetization transition region from becoming as large as that of the in-plane medium. Thus, with a much higher density of the magnetic recording disk device in mind, high expectation is now placed on the potential of a perpendicular magnetic recording system.
With regard to the perpendicular magnetic recording medium, there are largely two, i.e., a single-layered perpendicular medium and a double-layered perpendicular medium. The double-layered perpendicular medium includes a soft magnetic layer between a magnetic layer and a substrate for saving information, and the presence of this soft magnetic layer is a main difference from the single-layered perpendicular medium. Each medium has advantages and disadvantages. At present, however, the combined system of the double-layered perpendicular medium with a single magnetic pole head is most promising.
As a practical problem, also in the perpendicular magnetic recording medium, there is a problem of a reduction in read output, caused by the thermal fluctuation of magnetization. To solve this problem, it is important to enhance perpendicular orientation of an easy magnetization axis (c axis) regarding the magnetic layer mainly containing Co having a hexagonal close-packed structure (h. c. p.).
With regard to the conventional perpendicular medium, a technology has been proposed to provide a non-magnetic underlayer having an h. c. p. structure between the substrate and the magnetic layer, to enhance perpendicular orientation. An example is a TiCr underlayer (Journal of Magnetism and Magnetic Materials, 193, pp. 253-257 (1999)). A main element of the TiCr underlayer is Ti, obtained by adding 20 at. % or lower of Cr. A crystal is grown in such a way as to set the c axis of the underlayer perpendicular to the film surface, and by growing a magnetic layer thereon in a heteroepitaxial manner, the perpendicular orientation of the easy magnetization axis of the magnetic layer can be enhanced. Other than the non-magnetic h. c. p. underlayer, a technology for providing an underlayer containing Pt has been proposed (Journal of Magnetics Society of Japan, 24, pp. 267-270 (2000)).
To counter the thermal fluctuation of the perpendicular magnetic recording medium, it is important not only to enhance the perpendicular orientation of the axis of the easy magnetization, but also to increase squareness at least to 0.9 or more simultaneously. There are two methods of obtaining squareness, i.e., one by VSM measurement based on Mr/Ms in M-H loop, and the other by Kerr effect measurement. Regarding the foregoing TiCr underlayer, one satisfactory to a certain level in terms of perpendicular orientation can be obtained. Because of small squareness, however, the influence of thermal fluctuation is large, causing a reduction in read output. On the other hand, regarding the Pt underlayer, there is a difference in size from the crystal lattice of the magnetic layer though (111) orientation of the Pt underlayer is strong. Consequently, lattice matching is bad, making it difficult to improve perpendicular orientation as expected.
Further, to achieve a high recording density for the perpendicular magnetic recording medium, a reduction in medium noise becomes an important technical subject. An effective way of achieving low noise is to reduce the crystal grain size of the magnetic layer. Because of the heteroepitaxial growth of the magnetic layer on the underlayer, to micronize the crystal grain size of the magnetic layer, needless to say, the micronization of the gain size of the underlayer becomes an important technical subject.
Compared with the conventional medium, the medium of the present invention has a complex layer structure. Thus, the underlayer that has been described will be referred to as a non-magnetic intermediate layer hereinafter for convenience.